The present invention relates to a preparation of polyvinyl alcohol film, and more particularly to a method for preparing a biaxially oriented polyvinyl alcohol film having excellent tensile strength, impact strength and dimensional stability in which simultaneous biaxial stretching is effected by means of inflating an extruded tubing of the polymer.
It is well known that a polyvinyl alcohol film is excellent in transparency, toughness, flexibility, including anti-fogging and anti-static properties, and hitherto, a non-stretched polyvinyl alcohol film has been widely employed in various uses including wrapping materials. Also, it is expected that the demand for the film will further increase in place of a plasticized polyvinyl chloride film which causes serious problems in hygienic and incineration treatment, since the polyvinyl alcohol film does not generate chlorine or hydrochloric acid when burned.
However, a non-stretched polyvinyl alcohol film is poor in impact resistance, and is especially fragile at low temperature, unless a polyhydric alcohol such as glycerin, diethylene glycol, dipropylene glycol or sorbitol, or urea is added thereto as a plasticizer in comparatively large quantities, for instance, in an amount of 10 to 15 % by weight per polyvinyl alcohol. Therefore, since such a plasticizer is incorporated into a non-stretched polyvinyl alcohol film in comparatively large quantities in practical use, such a non-stretched film has the disadvantage that not only is the dimensional stability poor but printed adjacent colors appear out of register during the printing process due to film elongation at the time of converting to secondary products such as printing or bag making, and also the anti-blocking or slipability properties of the film are poor due to bleeding of the plasticizer.
In the case of a polyester film or a polypropylene film, biaxial stretching of such a film is carried out in order to improve the dimensional stability and impact resistance thereof. However, it is very difficult to biaxially stretch a polyvinyl alcohol film in the usual manner and, therefore, a commercially successful biaxially stretched polyvinyl alcohol film has never been manufactured. That is to say, when a polyvinyl alcohol film is stretched by means of a two-stage, successive biaxial stretching method, which is generally employed in biaxial stretching of a polyester or polypropylene film, (where the film is first stretched in a machine direction and then stretched in a transverse direction by employing a tenter), the film cannot be uniformly stretched in the transverse direction and frequently breaks at the second stage, since fibrillation and crystallization of the film take place at the first stage and, therefore, the successful production of such a film has been attended with many difficulties.
In general, presently available non-stretched polyvinyl alcohol films have a tensile strength within the range of 400 to 1,200 kg./cm..sup.2 and an aleongation within the range of 100 to 300 %.
It is reported in Japanese Patent Publication No. 32959/1970 that a uniaxially oriented polyvinyl alcohol film has a tensile strength of more than 1,200 kg./cm..sup.2 and an elongation of less than 80 % along the axis of orientation. However, an industrially produced biaxially oriented polyvinyl alcohol film having such mechanical properties in each of the machine and transverse directions has never been known.